1. Field of the Invention
The present invention relates to a positive electrode active material for a nonaqueous electrolyte secondary battery such as a lithium ion secondary battery (hereinafter, may also be simply referred to as “positive electrode active material”), a positive electrode mixture for a nonaqueous electrolyte secondary battery (hereinafter, may also be simply referred to as “positive electrode mixture”), and a nonaqueous electrolyte secondary battery. The present invention more specifically relates to a positive electrode active material, a positive electrode mixture, and a nonaqueous electrolyte secondary battery, having significantly improved battery characteristics and employing a lithium-transition metal composite oxide of a spinel structure.
2. Description of the Related Art
A nonaqueous electrolyte secondary battery has a feature that an operating voltage and an energy density are high as compared with a conventional nickel-cadmium secondary battery and the like, and has been widely used as a power source for electronic appliances. A positive electrode active material of the nonaqueous electrolyte secondary battery includes lithium-transition metal composite oxides represented by LiCoO2, LiNiO2, and LiMn2O4 and the like.
Of those, LiMn2O4 contains manganese existing in large amount as a resource, thereby employing lowly priced and easily available raw materials for LiMn2O4. There is another feature that environmental load is low. Further, a crystal structure of LiMn2O4 is stable even if all Li ions desorb from the crystal structure through a deintercalation reaction. Thus, a nonaqueous electrolyte secondary battery employing LiMn2O4 generates less heat in an over discharge state compared to those employing LiCoO2 and LiNiO2.
The nonaqueous electrolyte secondary battery employing LiMn2O4 having the advantages described above has been conventionally used for mobile electronic appliances such as cellular phones, laptop personal computers, and digital cameras, providing sufficient battery characteristics for such applications.
However, there is an increasing demand for improved characteristics of the mobile electronic appliances resulting from high-functionalization such as provisions of various functions, uses in high temperatures or low temperatures, or the like. Further, application of the nonaqueous electrolyte secondary battery to power sources such as batteries for electric vehicles is expected.
Therefore, sufficient battery characteristics cannot be obtained with a conventional nonaqueous electrolyte secondary battery, and further improvements are required.
JP 2001-6678 A (the term “JP XX-XXXXXX A” as used herein means an “unexamined published Japanese patent application”) describes a use of a lithium manganese oxide, as a positive electrode, prepared by modifying particles of a lithium manganese spinel compound by a compound of a different metal excluding lithium and manganese. JP 2001-6678 A describes that this lithium manganese oxide significantly improves a capacity damping rate at high temperatures.
However, sufficient cycle characteristics and high rate characteristics could not be obtained at normal temperatures using this lithium manganese oxide.
Further, JP 2000-90933 A describes a use of LiMZMeX-ZOY (where, M represents a substitution element, M≠Me, and Z represents an amount of substitution), as a positive electrode active material, prepared by substituting a part of a transition element Me in a lithium-transition element composite oxide, LiMeXOY, with two or more kinds of elements selected from the group consisting of Li, Fe, Mn, Ni, Mg, Zn, B, Al, Co, Cr, Si, Ti, Sn, P, V, Sb, Nb, Ta, Mo, and W. JP 2000-90933 A describes that this positive electrode active material improves electron conductivity of the positive electrode active material itself and reduces internal resistance of the battery while improving reversibility of a crystal structure of LiMn2O4 through intercalation/deintercalation of Li+, to thereby improve cycle characteristics as a battery. Further, JP 2000-90933 A describes that inclusion of at least Ti as a substitution element M is preferable for providing a notable effect of improving the electron conductivity and that Ti is preferably used for effectively preventing decrease of a positive electrode capacity.
However, sufficient battery characteristics could not be obtained using this positive electrode active material under an even harsher environment for use and specifically under high temperatures. Further, a sufficient charge-discharge capacity and sufficient high rate characteristics also could not be obtained.